The Third Generation Partnership Project (3GPP) Release 8 specifies for circuit switched fallback (CSFB) when a circuit switched (CS) transmission is delivered to a device camped on the long term evolution (LTE) network since LTE is a packet switched (PS) data based technology. By using CSFB, the device may select to a third generation (3G) radio access technology (RAT), such as the Universal Mobile Telecommunications System (UMTS), or a second generation (2G) RAT, such as the Global System for Mobile Communications (GSM), in order to receive the incoming CS transmission. Put differently, when an LTE device is used to make or receive a CS transmission such as a voice call or a short message service (SMS) text message, the LTE device will fall back to a 3G or 2G RAT in order to complete the CS transmission.
The LTE device will typically fall back to a 2G or 3G RAT network (RAN) (2G and 3G collectively, hereinafter, referred to as a “legacy RAT”) on a frequency indicated by the network. However, when the fall back to the legacy RAT on the indicated frequency fails, the device may take an extended period of time in order to connect to an appropriate RAT for reception of the CS transmission.
CSFB redirection to other RATs in the present mobile telecommunication system requires the mobile device, or user equipment (UE), to attempt camping on a suitable cell according to redirectedCarrierInfo if it is included in the RRCConnectionRelease/RRCConnectionReject message. However, for cases where the UE does not find a suitable cell on the target frequency, a search for any other suitable frequency in the target RAT could take a long time and might result in a CFSB mobile originated/mobile terminated (MO/MT) call drop. In addition, during the entire search process, the end user experiences a stalled data connection.